1. Field of the Invention
The present invention relates generally to an electric power steering system having an electric motor adapted to supply power steering assist directly to the steering system to thereby reduce the steering torque to be exerted by a driver, and more particularly to such a steering system in which noises and parasitic oscillations in the steering system are prevented.
2. Description of the Related Art
An electric power steering system is known in which an electric motor is driven by a target current generated based on a steering torque detected by a steering torque sensor and a vehicle velocity detected by a vehicle velocity sensor.
In another known electric power steering system, actual electric current flowing through an electric motor is fed back to a target current so that the deviation between the target current and motor current can be compensated for by means of a proportional plus integral (PI) control, or the deviation and the phase delay in the control system can be compensated for by means of a proportional plus integral plus derivative (PID) control.
Still another electric power steering system is known in which a low-pass filter is provided in a feedback loop of a motor current to attenuate gain in a high-frequency band to thereby eliminate high-frequency components and noises contained in the motor current.
The overall arrangement of a known electric power steering system is shown in FIG. 6, and dominant part of a known electric power steering system is shown in FIG. 7 in a block diagram.
As shown in FIG. 6, the electric power steering system, generally designated by reference numeral 1, includes: a rack-and-pinion mechanism 5 having a steering wheel 2, a steering shaft 3, a hypoid gear 4, a pinion 5A, a rack shaft 5B and so forth; a tie-rod 6; a steerable front wheel 7; an electric motor 8 for providing steering assist; a steering torque sensor 10 for sensing a steering torque to be applied to the steering wheel 2 and converting the sensed value into an electrical output torque signal T; a vehicle velocity sensor 11 for sensing a vehicle velocity and converting the sensed value into an electrical output vehicle velocity signal V; a control block 12 for driving and controlling the electric motor 8 in response to the steering torque signal T and the vehicle velocity signal V; a motor driver 13; and a motor current detector 14 for detecting motor current.
Upon operation of the steering wheel 2, the steering torque sensor 10 disposed on the steering shaft 3 senses a steering torque and converts the same into a corresponding electrical signal, which will then be transmitted to the control block 12 in the form of a steering torque signal T.
The rotational motion of the steering shaft 3 is transformed into a linear motion at the rack-and-pinion mechanism 5 via the pinion 5A in meshing engagement with the rack shaft 5B and is then transmitted through the tie-rod 6 to the front wheel 7 to steer the same.
The vehicle velocity sensor 11 senses a vehicle velocity and generates a corresponding electrical signal to be fed as a vehicle velocity signal V, to the control block 12.
The control block 12 supplies a motor control signal V.sub.o to a motor driver 13 in response to the steering torque signal T and the vehicle velocity signal V. The motor driver 13 drives the electric motor 8 by supplying motor voltage V.sub.M corresponding to the motor control signal V.sub.o.
The electric motor 8 driven by the motor voltage V.sub.M supplies steering assist to the steering system through the hypoid gear 4 to thereby reduce the steering torque to be exerted to the steering wheel 2 by a driver.
Motor current I.sub.M which flows through the electric motor 8 corresponds to motor voltage V.sub.M obtained, for example, by pulse width modulation (PWM) techniques.
Referring now to FIG. 7, the control block 12 of the power steering system 1 includes a target current generator 15 which outputs a target current I.sub.MS in response to the steering torque signal T detected by the steering torque sensor 10 and the vehicle velocity signal V detected by the vehicle velocity sensor 11.
The target current generator 15 stores data representative of steering torque (T)-target current(I.sub.Ms) characteristic where the vehicle velocity V shown in FIG. 8 is a parameter.
The control block 12 also includes a subtracter 16 which performs an operation as to the deviation .DELTA.I(=I.sub.MS -I.sub.MO) between the target current I.sub.MS and feedback current I.sub.MO resulted from filtering high-frequency components of the motor current I.sub.M.
The control block 12 further includes a drive controller 17 having a proportional plus integral (PI) control which generates a motor control signal V.sub.o for performing a pulse width modulation (PWM) control over the motor driver 13, and a low-pass filter 18 which is disposed in a feedback loop from the motor current detector 14 for removing high-frequency components from detected motor current I.sub.MD.
The proportional plus integral (PI) control of the drive controller 17 has a proportional element (P) and an integral element (I). The gain G and phase angle .theta. of their transfer function F (j.omega.) are represented by the Bode diagram of FIG. 10.
Referring to FIG. 10, it is known that there occurs a phase delay (.theta.=-90.degree.) at areas of low angular frequency .omega. while the gain G (=20logG) may be improved significantly. By contrast, the gain G (=20logG) is low at areas of high angular frequency .omega. while the phase delay can be improved significantly.
The low-pass filter 18 may be any primary CR (resistor, condenser) low-pass filter, e.g., the one shown in FIG. 9, which is designed to reduce noises present in high angular frequency components and gains in high-frequency bands, without imparting any influence upon the steering system, by removing high-frequency components at an attenuation rate of 6 dB per octave in the case of frequencies higher than a cutoff angular frequency of .omega.ca {=1/(C.times.R1)}.
The known electric power steering systems thus arranged have an advantage in that it is possible to significantly improve the gain G at areas of low angular frequency .omega. and phase delay at areas of high angular frequency .omega. and to remove noises of high angular frequency. However, the low-pass filter of the systems has an inconvenience such that the phase as the angular frequency becomes higher. Further, in the feedback loop of the control block 12, if the phase turns by 2 .pi., the resultant loop gain shall be larger than 1, whereby oscillation conditions are met and noises are generated due to parasitic oscillations, thus affecting the steering assist.
One of ordinary skill in the art may consider altering the proportional element for the proportional plus integral control and an integral element coefficient so as to prevent oscillations. However, this may not be an acceptable solution in that where each coefficient is small, the desired steering assist may not always be obtained and in that where a large coefficient is chosen as the proportional element, the focusing of steering characteristic may deteriorate and the steering feel may be adversely affected.